In recent years, from the standpoint of global warming and the like, hydrogen is attracting attention as a clean energy involving no formation of carbon dioxide during combustion.
In general, for the production of hydrogen, steam reforming of a hydrocarbon fuel, represented by the following formulae (A1) and (A2), is employed:CnHm+nH2O→nCO+(n+m/2)H2  (A1)CO+H2O→CO2+H2  (A2)Total reaction:CnHm+2nH2O→nCO2+(2n+m/2)H2 
Accordingly, although carbon dioxide is not formed by the combustion itself of hydrogen, carbon dioxide is generally formed in the production of hydrogen.
In this connection, use of solar thermal energy or nuclear thermal energy has been proposed as the method for producing hydrogen without using a hydrocarbon fuel (see, Patent Document 1 and Non-Patent Document 1).
As the method for producing hydrogen from water by utilizing thermal energy, there has been proposed a method called an I—S (iodine-sulfur) cycle process represented by the following formulae (B1) to (B3):H2SO4(liquid)→H2O(gas)+SO2(gas)+1/2O2(gas)  (B1)                (reaction temperature=about 950° C., ΔH=188.8 kJ/mol-H2)I2(liquid)+SO2(gas)+2H2O(liquid)→2HI(liquid)+H2SO4(liquid)  (B2)        (reaction temperature=about 130° C., ΔH=−31.8 kJ/mol-H2)2HI(liquid)→H2(gas)+I2(gas)  (B3)        (reaction temperature=about 400° C., ΔH=146.3 kJ/mol-H2)        
The total reaction of the I—S (iodine-sulfur) cycle process represented by formulae (B1) to (B3) is as follows:H2O→H2+1/2O2 
(ΔH=286.5 kJ/mol-H2 (on the basis of higher heating value))
(ΔH=241.5 kJ/mol-H2 (on the basis of lower heating value))
The reaction of formula (B1) can be divided into two elementary reactions of the following formulae (B1-1) and (B1-2):H2SO4(liquid)→H2O(gas)+SO3(gas)  (B1-1)                (reaction temperature=about 300° C., ΔH=90.9 kJ/mol-H2)SO3(gas)→SO2(gas)+1/2O2(gas)  (B1-2)        (reaction temperature=about 950° C., ΔH=97.9 kJ/mol-H2)        
That is, in the case of producing hydrogen by an I—S cycle process, the sulfur trioxide (SO3) decomposition reaction of formula (B1-2) requires a highest temperature, and the high temperature required in this reaction cannot be easily obtained.
Non-Patent Document 1 describes that, with respect to such a problem, a natural gas is burned as needed while using solar thermal energy as the heat source, and thereby additional thermal energy is obtained.
Also, in order to lower the temperature required in the sulfur trioxide decomposition reaction of formula (B1-2), use of a platinum catalyst has been proposed. It is known that a platinum catalyst may have high performance at the start of use in this reaction; however, the catalytic activity is reduced due to of oxdization of the platinum with oxygen produced in the reaction, and due to sintering of the platinum particle. Furthermore, the platinum catalyst is expensive, and therefore its use on an industrial scale is difficult.
In this connection, Non-Patent Document 2 has proposed a technique where, in order to lower the temperature required in the sulfur trioxide decomposition reaction, a catalyst selected from the group consisting of platinum (Pt), chromium (Cr), iron (Fe) and oxides thereof is used by depositing it on an alumina support.
In addition, with respect to the I—S cycle process, Patent Document 2 has proposed a technique where, in the reaction represented by formula (B2), that is, in the reaction of obtaining hydrogen iodide and sulfuric acid from iodine, sulfur dioxide and water, the reaction of sulfur dioxide with water is performed on the cathode side of a cation-exchange membrane, and the reaction of iodine is performed on the anode side of the cation-exchange membrane, and thereby the subsequent separation operation is omitted.
Incidentally, other than the I—S cycle process, a Westinghouse cycle process, an Ispra-Mark 13 cycle process and the like are known as the method for producing hydrogen by utilizing thermal energy. However, also in these processes, sulfur trioxide must be decomposed into sulfur dioxide and hydrogen as in formula (B1-2).